This invention is a humidifier used to increase and maintain the humidity of a musical instrument, such as a guitar. The humidifier is a container made of a water vapor permeable material that contains a water absorbent material such as acrylate polymer. The container is held within the body of the instrument by being mounted onto the strings of the instrument. The container is filled with water by an opening that is sealed with a cap. The water absorbent material can hold water without dripping, but will allow the water to evaporate.